Lithium-air (Li-air) batteries are currently the subject of intense scientific investigation due to the extremely high theoretical energy density of 12 kWh/kg, which far exceeds that of any other existing energy storage technology.
In Li-air batteries, the oxygen electrode should be porous. The porosity stores the solid products generated from the reaction of Li ions with O2 (e.g. Li2O and Li2O2) during the discharge cycle of the battery, and porous oxygen electrode must integrate a catalyst to promote the reactions. A variety of factors dictate the extent of electrochemical (discharge and charge) reactions in Li-air cells including the nature of the catalyst, the catalyst distribution on the porous cathode, the pore volume of the cathode, as well as the type of the applied organic electrolytes.
Both the surface area and porosity of the cathode are critical for the performance of lithium-air batteries. Larger surface areas provide more catalyst particles and catalytically active sites to accelerate the electrochemical reactions and increase the current density on discharge. Larger pores facilitate faster oxygen diffusion and provide the volume necessary to accommodate the reduction products deposited during discharge.
For practical applications, Li-air batteries must be rechargeable, thereby necessitating using a sufficiently high potential or a catalyst to promote the discharge reactions. However, high overpotentials on charge and discharge, even at very low current densities of 0.01-0.05 mA/cm2, result in very low cycling efficiencies (<60%) and low power capability.
Metals, metal complexes, and metal oxides have all been examined as the cathode catalysts in the Li-air cells, and these catalysts show large differences in discharge capacity and charge plateau. For instance, the charging activity of pure carbon is very poor, showing an average voltage plateau at 4.7 V. This plateau is slightly reduced when a catalyst is introduced to the cathode. For example, the charge plateau was reduced to 4.2 V using a MnOx catalyst on the carbon support and allowed a charging density of 0.1 mA/cm2. However, it should be pointed out that the charge overpotential is still high (>4.0 V) in most of cases despite different catalysts are applied to the cathode.
Although much of the reported research has been focused on developing metal catalysts and cathode structures, it has recently become apparent that the electrolyte plays a key role in the cell performance. The oxygen anion radical O2− intermediate or other reduction species, which may be formed during the discharge process, can be highly reactive and may cause the electrochemical response to be dominated by electrolyte decomposition rather than by the expected lithium peroxide formation. However, it should be recognized that the carbon cathode structure (together with the catalyst) is coupled with the electrolyte in the Li-air battery. For instance, the decomposition of electrolyte may takes place on the carbon surface due to its natural defects on the carbon, which serves as the catalytic site not only for the formation of the desired discharged products (Li2O2), but also for the decomposition of electrolytes.
We applied Atomic layer deposition (ALD) technique as an example to demonstrate the effect of coating of porous carbon on the efficiency of Li-air battery. Atomic layer deposition (ALD) is a technique for preparing thin films on planar substrates that employs self-limiting chemical reactions between gaseous precursors and a solid surface allowing atomic scale control over the film thickness and composition. One of the distinguishing attributes of ALD is the capability to deposit highly uniform and conformal coatings on surfaces with complex topographies and to infiltrate mesoporous materials. This feature is particularly attractive for the synthesis of heterogeneous catalysts requiring highly dispersed catalytic species on high surface area, mesoporous supports. Consequently, ALD is being explored as an alternative method for preparing advanced catalysts.
The layer-by-layer growth process afforded by ALD typically yields smooth, uniform films and this is ideal for most microelectronics applications. However, non-uniform deposits can occur when the ALD chemistry is inhibited on the starting substrate or when the ALD material aggregates from surface diffusion. Both of these mechanisms are in effect in the early stages of noble metal ALD on oxide surfaces, which result in the formation of discrete, three-dimensional nanoparticles decorating the surface. This behavior has been exploited to synthesize supported noble metal catalysts exhibiting remarkably high activity as a result of the highly dispersed, small noble metal particles. The good dispersion of the active particles on the support during ALD enables a decreasing of the metal loading while still achieving the same catalytic activity as the catalysts with higher metal loading prepared by other methods. This is especially important with noble metal materials where the excess use of the raw materials should be avoided. Uniform palladium nanoparticles in the size range from sub-nanometer to a few nanometers, are one of the most efficient catalysts for facilitating the oxygen reduction reaction (ORR) in the fuel cell, and have been synthesized by ALD on high surface area supports. However, performing ALD on porous carbon surface in general has been a technical challenge because of the lack of active sites on carbon for surface reactions, and therefore, has not been well studied.